The invention relates to a double-sided printed circuit board.
In the event of an abnormal operating state, in particular in the event of a short circuit, the conductor track fuse of a printed circuit board has the task of rapidly and reliably interrupting the connection to the voltage supply. This is intended to prevent the short circuit from propagating on the printed circuit board and incurring further damage. Moreover, the early response of the conductor track fuse is intended to prevent a power supply circuit-breaker from being triggered.
For this purpose, a specific conductor track section on the printed circuit board is often embodied as a fusible link. This conductor track section has a cross section which is constricted in comparison with the other conductor tracks on the printed circuit board and which is intended to melt in a controlled fashion and thus to interrupt the electric circuit when a short circuit occurs.
One problem in this case is that the plasma occurring during the melting and evaporation of the fusible link finds a new pair of contacts with low impedance under power supply voltage and thus continues to burn in an uncontrolled fashion. In this way, an indirect short circuit is triggered which in most cases burns between soldered joints and/or significantly wider conductor tracks and, on account of the greater amount of metal available, generates significantly more conductive plasma than the actual short circuit and thus causes further damage on the printed circuit board. The process escalates until finally the power supply circuit-breaker is triggered. The lower the impedance of the downstream power supply filter, that is to say the higher the rated power of the device, the more frequently the case described above occurs.
An attempt to solve this problem is found in EP 0 774 887 A2. The latter discloses a conductor track fuse in which two conductor track sections, through which current flows in opposite directions, are arranged as parallel as possible at a short distance from one another, such that the conductor track overall has a U shape. Owing to the short distance, the magnetic fields interact with one another in the region of the conduction sections. One of the conductor track sections has a constricted cross section and melts in the case of a short circuit. The resulting plasma is kept away from the adjacent conductor section owing to the interaction of the magnetic fields and the plasma can be prevented from continuing to burn.
However, the problem outlined in the introduction cannot be solved reliably and for every application by means of the abovementioned attempt. Primarily in the case of densely populated printed circuit boards, there is the risk of the plasma jumping over to other adjacent conductor tracks in which there is as yet no short-circuit current flowing and which accordingly also do not have a magnetic field (of whatever polarity) strong enough for repelling the plasma. Moreover, for reasons of space, it will not always be possible to accommodate this U-shaped conductor track fuse on every printed circuit board.
DE 37 23 832 A1 discloses a conductor track fuse in which a fusible link is likewise used, which has a conductor track cross section reduced by a cutout in this case. This location having the reduced cross section is covered with a spark-quenching medium. Moreover, the cutout in the fusible link, which can be e.g. a hole and can extend through to the printed circuit board, is intended to enable the gases that arise in the course of melting to flow away.
What is problematic about this solution is the complicated application of the covering layer and production of the cutout in the conductor track. Furthermore, a heat accumulation having an unfavorable effect on the interruption of the short-circuit current can arise underneath the covering layer.